A NATION CHALLENGED: AN INQUIRY; F.B.I. Asks if Hijacking Plot Included Plane at Kennedy

The Federal Bureau of Investigation is looking into whether a United Airlines flight scheduled to leave Kennedy International Airport for San Francisco was a target of hijackers on Sept. 11, aviation and law enforcement officials said.

The United plane, Flight 23, was late pushing back from the gate and was seventh in line for takeoff when Kennedy was closed because of the terrorist attacks.

When the captain announced that the plane was returning to the gate, four male passengers described as Middle Eastern in appearance stood up and began urgently consulting with each other, two aviation officials said. The men refused orders from flight attendants to return to their seats, the officials said.

The cockpit crew did not wait for the men to be seated but decided to taxi back to the gate anyway, the officials said. ''As soon as the door opened, these four guys bolted,'' said one of the officials. The other added that the F.B.I. later interviewed the crew about the incident.

Law enforcement officials confirmed that the F.B.I. is investigating the incident. A United spokesman refused to comment, except to say that the passenger list had been turned over to investigators.

Like two of the four planes that were hijacked that day, the Kennedy plane was a Boeing 767, and like all four, it was scheduled for a transcontinental flight. Its scheduled departure time, 8:30, was within the time range of the four hijacked flights.

The question of whether hijackers had intended to take over more than four flights on Sept. 11 has been the subject of confused speculation in the last few days. On Wednesday, the head of the Federal Aviation Administration, Jane F. Garvey, said in a speech that her agency's quick grounding of commercial air traffic nationwide had thwarted other hijackings. But later in the day she said she had no evidence of other planned attacks and was relying partly on news reports of knives being found on other airplanes.

Other flights using Boeing 757's and 767's are under scrutiny because all four planes hijacked were of those two models. On the morning of Sept. 11, when the number of planes hijacked was still not clear, federal officials took a quick count of 757's and 767's still airborne and told air-traffic controllers to watch them for any unusual behavior.

In the days after Sept. 11, rumors and reports have swept the F.A.A. and airlines about possible additional targets, including an Air Canada flight scheduled to go from Toronto to La Guardia Airport, and an American Airlines flight at Kennedy waiting to take off for London, although the evidence for those cases appears to be circumstantial.

The Air Canada plane was scheduled to fly from Toronto to La Guardia but was caught on the ground by the closing of American airspace. On Sept. 14, when the plane was scheduled to fly to Calgary, Alberta, a passenger spotted two knives in an overhead baggage compartment.

But speaking to reporters in Washington on Tuesday, Herb Gray, the deputy prime minister of Canada, said that it was not clear when the knives were put on the plane, and that ''they weren't the big box cutters'' believed to be used by the hijackers of the other planes.

An Air Canada spokesman described them as ''X-acto-type knives,'' saying that they had small, retractable blades.

The Air Canada plane was impounded by the Royal Canadian Mounted Police but then released. It was an Airbus A-320, which is considerably smaller than the 757's and 767's that were hijacked, and has a far different control system.

Some aviation officials also believe that American Airlines Flight 142, which was due to take off from Kennedy for London at 8:30 a.m. on Sept. 11, was also a target.

That plane was delayed at the gate when the cockpit crew asked for a mechanic to check a valve in an engine. When a delay was announced, one passenger asked to leave, so the cabin door was opened for him and closed again. Then, according to aviation officials, a second passenger also demanded to leave, and became ''rude and indignant'' when the door could not be opened immediately.

A cockpit crew member wanted to alert the F.B.I. immediately but did not know the passenger's name, the officials said. The flight attendants knew the man's identity, but they had been escorted out of the airport with the passengers.

That plane was a Boeing 777, which would have had an explosive force similar to a 767, but has a slightly different cockpit from the planes that were hijacked.